I just started playing 'nids, and so I had been wondering about this myself. After reading the very limited rules for assaulting in ruins, and all of the other pertinent information for moving through levels in ruins, my interpretation is that you don't need to be in base contact, as long as you COULD make the assault move.
For instance, a Trygon is trying to assault some Necrons on the second level of a ruin. There is no way that a Trygon, or pretty much any
MC that is not a C'tan, would have any hope in fitting on the floor of a ruin. So in the
BRB, page. 85, it says:
...or the space could be very limited on a particular level, making it impossible to move assaulting models into base-to-base contact with the unit they wish to assault. When this happens, it is perfectly acceptable to place models as close to their foe as possible, including the level below or above, providing you put the assaulting models as close as possible to their opponents...
The same section also indicates you can use unit coherency rules to determine who can assault, as in who is within 2" of someone who is in 'base contact'.
So, the way I view the above scenario is this:
In order for the Trygon to be able to assault, he needs to make a difficult terrain test (on
3D6 for Move Through Cover), and the distance would need to be enough to make it to the base of the ruins, and go up the 3". So, you and your opponent determine how far you have to go in order to assault, and then make said difficult terrain check. If you get the required distance, anywhere from 3-6", you would move the trygon as close as possible to the necrons. They then need to do a defender's react, and really can be on either the ground floor with the Trygon, or on the second floor, in unit coherency with people in base contact on the first floor.
Don't get me wrong, I think this is open to a lot of interpretation, but it is absolutely ridiculous to think that hiding on the second floor of a ruin would completely negate the ability of a monstrous creature to attack you. There would be no reason for a shooty army to ever do anything besides sit on ruins, and laugh at
MCs wandering around aimlessly below them, even if the model's head goes above the second level.
Oh, and I agree, no tournament play for me because of things like this. Me and my buddies know you have to be fairly open to interpretation when dealing with bizarre scenarios, but combine the obvious intent of the rules with common sense, and you are good to go.